Network Working Group Paul Mockapetris
Request for Comments: 973 ISI
January 1986
Domain System Changes and Observations
STATUS OF THIS MEMO
This RFC documents updates to Domain Name System specifications
RFC-882 [1] and RFC-883 [2], suggests some operational guidelines,
and discusses some experiences and problem areas in the present
system. Distribution of this memo is unlimited.
This document includes all changes to the Domain System through
January, 1986. Change notices and additional discussion are
available online in file [USC-ISIB.ARPA]DOMAIN.CHANGES.
OVERVIEW
This memo is divided into four major sections:
"UPDATES" which discusses changes to the domain specification
which are in widespread use and should be regarded as being part
of the specification.
"OPERATION GUIDELINES" which suggests rules-of-thumb for using the
domain system and configuring your database which are appropriate
in most cases, but which may have rare exceptions.
"EXPERIENCES" which discusses some unusual situations and common
bugs which are encountered in the present system, and should be
helpful in problem determination and tuning.
"PROBLEM AREAS" which discusses some shortcomings in the present
system which may be addressed in future versions.
UPDATES
This section discusses changes to the specification which are final,
and should be incorporated in all domain system software.
TTL timeouts too small
The 16 bit TTL field in RRs could not represent a large enough
time interval. The 16 bit field, using seconds for units, has a
maximum period of approximately 18 hours.
All time values, including all TTLs and the MINIMUM field of the
SOA RR, are expanded to 32 bits.
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RFC 973 January 1986
Domain System Changes and Observations
CLASS changes
Class 2, originally reserved for CSNET, is obsolete. Class 3 has
been assigned for use by CHAOS.
CNAME usage
The specification allows CNAME RRs to exist with other RRs at the
same node. This creates difficulties since the other RRs stored
with the CNAME at the alias might not agree with the RRs stored at
the primary name.
If a node has a CNAME RR, it should have no other RRs.
* semantics
The use of * to represent a single label wildcard, along with the
possibility of multiple * labels, led to difficult server
implementations and complicated search algorithms. There were
also questions regarding whether a * based specification could
refer to names that were not contained in the zone which had the *
specification.
While we might want the "inheritability" for some cases, it leads
to implementation difficulties. The first of these is that
whenever we can't find a RR in a particular zone, we have to
search all parent zones to look for a suitable * result.
(Alternatively we could develop some automatic method for insuring
consistency or insist on careful duplication of inherited data.)
We also must deal with conflicts, i.e. what if a subdomain doesn't
want to inherit defaults.
Given these difficulties, the solution is to insist that
delegation of authority cancels the * defaults. This is quite
simple to implement; all you need to do is to check for delegation
before looking for * RRs.
A second difficulty is the restriction that * match a single
label. Thus if a name server is looking for RRs for the name
A.B.C.D.E.F, it must check for *.B.C.D.E.F, *.*.C.D.E.F,
*.*.*.D.E.F, etc. This check must also be careful of zone
boundaries and multiplies the effort to handle a query.
The solution adopted is to allow a single * label in the leftmost
part of a name stored in a zone, and to allow this label to match
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Domain System Changes and Observations
any number of unknown labels or a single known label in the query
name. However, the * match is only taken for parts of the tree
which are neither delegated or explicitly represented.
The algorithm for performing the search in a tree structured
database has the following steps:
1) Descend in the tree matching labels from right to left. If a
delegation is found return that; if the specified node is found
go to step 2, if the tree ends go to step 3.
2) Look for RRs that answer the query. If any are found, return
them as the answer. If none are found, look for answers in a *
node which has the same name as the query name except for the
rightmost label. (e.g. if you can't find an answer at F.ISI.ARPA,
look for a RR at *.ISI.ARPA)
3) The search for a desired name has failed; look for a node whose
name is * plus however much matched. Look for answers there.
(e.g. If you are looking for X.Y.ISI.ARPA and the tree ends at
ISI.ARPA, look at *.ISI.ARPA. The same thing holds for
Y.ISI.ARPA, or any name of the form .Z.ISI.ARPA, where Z
is a label that doesn't exist under ISI.ARPA)
Note that this interpretation means that * matches names that are
not in the tree, no matter how much of the tree is missing, and
also matches one level's worth of known tree.
AA semantics
When a name server is responding to a query for a particular name
and finds a CNAME, it may optionally restart the search at the
canonical name. If the server uses the restart feature, the
answer section of the returned query contains one (or more)
CNAMEs, possibly followed by answers for the primary name. The
canonical name will usually be in the same zone as the alias, but
this need not be the case. If the server is authoritative for one
of the names but not both, it is not clear whether the AA bit
should be set.
The solution adopted is to make the AA refer to the original query
name.
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Domain System Changes and Observations
Master file format
The present specification uses a somewhat awkward method for
representing domain names in master files.
The change adopted is that all domain names in this file will be
represented as either absolute or relative. An absolute domain
name ends with a ".". A free standing "." is assumed to refer to
the root. A relative domain name doesn't end with a dot, and is
assumed to be relative to the current origin.
SERIAL number size
If the master file changes rapidly, an infrequently updated copy
may miss the wrapping of the sequence number in the SERIAL field
of the SOA, or misinterpret the number of updates that have taken
place.
The SERIAL field is increased to 32 bits.
MD and MF replaced by MX
The original specification uses MD and MF RRs for mail agent
binding. The problem is that a mailer making a MAILA query, which
asks for both types, can't use the cache since the cache might
have the results for a MD or MF query. That is, the presence of
one of these types of information in the cache doesn't imply
anything about the other type. The result was that either mailers
would have to always consult authoritative servers or try to use
partial information; neither of these is really acceptable.
The change is to replace MD and MF with a new type of RR called MX
which conveys similar information in a single RR type. MX has
been assigned a type code of 15 decimal. The format of the MX RR
is a 16 bit preference value followed by a domain name. A node
may have multiple MX RRs, and multiple MX RRs with the same
preference value are allowed at a given node.
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Domain System Changes and Observations
The preference values denote the relative preference that the mail
destination places on the mail agents, with lower values being
"better". A mailer is expected to at least try the mail agent(s)
with the lowest preference value. The significance of particular
preference values, the units of preference, and the linearity of
preference values are not defined but left open; preference values
should only be used to establish relative rankings.
For example, the current RRs:
MAIL-ORG MD HOST1
MD HOST2
MF HOST3
might be replaced by:
MAIL-ORG MX 10 HOST1
MX 10 HOST2
MX 20 HOST3
The values 10 and 20 have no significance other than 10<20. A
detailed discussion of the use of MX is the subject of [3].
Zone transfer
The original specification states that zone transfers take place
in breadth first order. The intent was to make the transfer
easier for the accepting name server to handle. This now doesn't
work out to be very helpful, and is a severe pain for implementers
using various hashing algorithms. The new rule is that you can
transmit the records in any order you choose, so long as the SOA
node of the zone is transmitted first and last, and no other
duplication occurs.
IN-ADDR domain renamed
The name of the IN-ADDR domain is now IN-ADDR.ARPA. This change
was made because many felt that the use of a top-level name was
inappropriate to network-specific information.
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Domain System Changes and Observations
OPERATIONAL GUIDELINES
This section suggests rules-of-thumb for using the domain system and
configuring your database which are appropriate in most cases, but
which may have rare exceptions.
Zone delegation
When a domain wishes to become independent from its parent, the
RRs which mark the delegation in the parent and child zones should
be carefully synchronized to minimize the possibility that
resolvers become confused.
For example, suppose that we wish to create a new zone called
ISI.EDU under an existing EDU zone, and that the servers for the
child zone are X.ISI.EDU and Y.GOV.
We might add the following to the parent zone:
ISI.EDU. 10000 NS X.ISI.EDU.
10000 NS Y.GOV.
X.ISI.EDU. 10000 A
Y.GOV. 10000 A
and the following to the child zone:
ISI.EDU. 10000 NS X.ISI.EDU.
10000 NS Y.GOV.
50000 SOA
X.ISI.EDU. 10000 A
Y.GOV. 10000 A
Note the following:
In both cases, the A RR for Y.GOV is included, even though
Y.GOV isn't in the EDU or ISI.EDU domains. This RR isn't
authoritative, but is included to guarantee that the address of
Y.GOV is passed with delegations to it. Strictly speaking this
RR need not be in either zone, but its presence is recommended.
The X.ISI.EDU A RR is absolutely essential. The only time that
a server should use the glue RRs is when it is returning the NS
RRs and doesn't otherwise have the address of the server. For
example, if the parent server also was authoritative for GOV,
the glue RR would typically not be consulted. However, it is
still a good idea for it to be present, so that the zone is
self-sufficient.
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Domain System Changes and Observations
The child zone and the parent zone have identical NS RRs for
the ISI.EDU domain. This guarantees that no matter which
server is asked about the ISI.EDU domain, the same set of name
servers is returned.
The child zone and the parent zone have A RRs for the name
servers in the NS RRs that delegate the ISI.EDU domain. This
guarantees that in addition to knowing the name servers for the
ISI.EDU domain, the addresses of the servers are known as well.
The TTLs for the NS RRs that delegate the ISI.EDU domain and
the A RRs that represent the addresses of the name servers are
all the same. This guarantees that all of these RRs will
timeout simultaneously. In this example, the value 10000 has
no special significance, but the coincidence of the TTLs is
significant.
These guidelines haven't changed any of the flexibility of the
system; the name of a name server and the domains it serves are
still independent.
It might also be the case that the organization called ISI wanted
to take over management of the IN-ADDR domain for an internal
network, say 128.99.0.0. In this case, we would have additions to
the parent zone, say IN-ADDR.ARPA.
We might add the following to the parent zone:
99.128.IN-ADDR.ARPA. 2000 NS Q.ISI.EDU.
2000 NS XX.MIT.EDU.
Q.ISI.EDU. 2000 A
XX.MIT.EDU. 2000 A
and the following to the child zone:
99.128.IN-ADDR.ARPA. 2000 NS Q.ISI.EDU.
2000 NS XX.MIT.EDU.
5000 SOA
Q.ISI.EDU. 2000 A
XX.MIT.EDU. 2000 A
SOA serials
The serial field of the SOA RR for a domain is supposed to be a
continuously increasing (mod 2**32) value which denotes the
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Domain System Changes and Observations
version of the database. The idea is that you can tell that a
zone has changed by comparing serial numbers. When you change a
zone, you should increment the serial field of the SOA.
All RRs with the same name, class, and type should have the same TTL.
The logic here is that all of them will timeout simultaneously if
cached and hence the cache can be reliably used.
Case consistency
The domain system is supposed to preserve case, but be case
insensitive. However, it does nobody any good to put both RRs for
domain name xxx and XXX in the data base - It merely makes caching
ambiguous and decreases the efficiency of compression. This
consistency should also exist in the duplicate RRs that mark
delegation in the delegator and delegatee. For example, if you
ask the NIC to delegate UZOO.EDU to you, your database shouldn't
say uzoo.edu.
Inappropriate use of aliases
Canonical names are preferred to aliases in all RRs. One reason
is that the canonical names are closer to the information
associated with a name. A second is that canonical names are
unique, and aliases are not, and hence comparisons will work.
In particular, the use of aliases in PTR RRs of the IN-ADDR domain
or in NS RRs that mark delegation is discouraged.
EXPERIENCES
This section discusses some unusual situations and common bugs which
are encountered in the present system, and should be helpful in
problem determination and tuning. Put differently, you should try to
make your code defend against these attacks, and you should expect to
be the object of complaint if you make these attacks.
UDP addresses
When you send a query to a host with multiple addresses, you might
expect the response to be from the address to which you sent the
query. This isn't the case with almost all UNIX implementations.
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UDP checksums
Many versions of UNIX generate incorrect UDP checksums, and most
ignore the checksum of incoming UDP datagrams. The typical
symptom is that your UNIX domain code works fine with other
UNIXes, but won't communicate with TOPS-20 or other systems.
(JEEVES, the TOPS-20 server used for 3 of the 4 root servers,
ignores datagrams with bad UDP checksums.)
Making up data
There are lots of name servers which return RRs for the root
servers with 99999999 or similar large values in the TTL. For
example, some return RRs that suggest that ISIF is a root server.
(It was months ago, but is no longer.)
One of the main ideas of the domain system is that everybody can
get a chunk of the name space to manage as they choose. However,
you aren't supposed to lie about other parts of the name space.
Its OK to remember about other parts of the name space for caching
or other purposes, but you are supposed to follow the TTL rules.
Now it may be that you put such records in your server or whatever
to ensure a server of last resort. That's fine. But if you
export these in answers to queries, you should be shot. These
entries get put in caches and never die.
Suggested domain meta-rule:
If you must lie, lie only to yourself.
PROBLEM AREAS
This section discusses some shortcomings in the present system which
may be addressed in future versions.
Compression and types
The present specification attempts to allow name servers and
resolvers to cache RRs for classes they don't "understand" as well
as to allow compression of domain names to minimize the size of
UDP datagrams. These two goals conflict in the present scheme
since the only way to expand a compressed name is to know that a
name is expected in that position.
One technique for addressing this problem would be to preface
binary data (i.e. anything but a domain name) with a length octet.
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Domain System Changes and Observations
The high order two bits of the length octet could contain either
01 or 10, which are illegal for domain names. To compensate for
the additional bytes of data, we could omit the RDATA length field
and terminate each RR with a binary length field of zero.
Caching non-existent names
In the present system, a resolver has no standard method for
caching the result that a name does not exist, which seems to make
up a larger than expected percentage of queries. Some resolvers
create "does not exist" RRs with TTLs to guarantee against
repetitive queries for a non-existent name.
A standard technique might be to return the SOA RR for the zone
(note that only authoritative servers can say name does not exist)
in the reply, and define the semantics to be that the requester is
free to assume that the name does not exist for a period equal to
the MINIMUM field of the SOA.
Cache conflicts
When a resolver is processing a reply, it may well decide to cache
all RRs found in sections of the reply. The problem is that the
resolver's cache may already contain a subset of these RRs,
probably with different TTLs.
If the RRs are from authoritative data in the answer section, then
the cache RRs should be replaced. In other cases, the correct
strategy isn't completely clear. Note that if the authoritative
data's TTL has changed, then the resolver doesn't have enough
information to make the correct decision in all cases.
This issue is tricky, and deserves thought.
REFERENCES
[1] Mockapetris, P., "Domain Names - Concepts and Facilities",
RFC-882, USC Information Sciences Institute, November 1983.
[2] Mockapetris, P., "Domain Names - Implementation and
Specification", RFC-883, USC Information Sciences Institute,
November 1983.
[3] Partridge, C., "Mail Routing and the Domain System", RFC-974,
CSNET-CIC, BBN Laboratories, January 1986.
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